(1) Field of the Invention
This invention relates to a filter medium comprising one fibrous web or a plurality of piled and integrated webs, which are comprised of ultra-fine combustible synthetic fibers three-dimensionally interlaced and entangled in various directions. This filter medium is excellent in filtering efficiency, has a long life and a high tenacity and can be burnt, and is particularly useful as an air filter. The present invention also relates to a process for preparing this filter medium.
(2) Description of the Prior Art
A high-efficiency air filter for use in filtering and purifying air containing radioactive dusts in a room in an atomic power plant or atomic power research institute where radioactive substances are handled and discharging purified air from the room, or a high-efficiency air filter for use in filtering and purifying exterior air and introducing purified air into a clean room in a hospital, a precision machine-assembling plant or a semiconductor-preparing plant, is required to collect ultra-fine dusts having a size smaller than 1.mu. at a high efficiency.
High-efficiency air filters heretofore used for the above-mentioned purposes comprise as a filter medium a non-woven fabric obtained from cut fibers of fine glass filaments according to the paper-making method. There has not been known a high-efficiency air filter comprising a filter medium composed of fibers other than glass fibers.
A high-efficiency air filter to be used for the above-mentioned purposes is required to have such a property that 99.0% or more of ultra-fine particles having a size of 0.3.mu. be collected at a small air permeation resistance, such as less than 50 mmH.sub.2 O as measured at an air speed of 2.3 cm/sec. This requirement has not been satisfied by any air filters other than a high-efficiency air filter comprising a filter medium formed from ultra-fine glass fibers according to the paper-making method.
In this filter medium formed from glass fibers according to the paper-making method, however, since the constituent fibers are arranged only two-dimensionally, collection of ultra-fine dusts can be done only on the surface of the filter medium, and therefore, it is very difficult to increase the amount collected of dusts. Furthermore, because of a low tenacity due to the brittleness of glass fibers, the filter medium is readily broken during the handling or use. Moreover, since glass fibers are incombustible, disposal of the filter medium by burning is impossible, and the glass fiber filter medium involves a serious problem, that is, how to discard the used filter.
Since a used high-efficiency air filter containing radioactive dusts cannot simply be discarded, it is packed and sealed in a vessel such as a drum can, as it is or after compression and is stored in this state, and it is a problem how to maintain a place for the storage of these packed vessels.
Accordingly, it has eagerly been desired to develop a filter medium valuable for a combustible high-efficiency air filter which is not broken during the handling or use, has an efficiency of collecting ultra-fine dusts, has a long life and can be burnt.